How and Why We Came to Sequence Northern White Rhino Genomes

What is astounding, and hopeful, is that the frozen cell cultures banked in the Frozen Zoo® represent a significant sampling of the genetic diversity of northern white rhinos and a potential means for preventing extinction of this form of rhino. From our first northern white rhino cell culture established over 35 years ago, through the last northern white rhino calf, born in 2000 and added to the Frozen Zoo in December 2009, there is more of the gene pool of these rhinos in the Frozen Zoo than survives in the living animals. Given the dire situation, we are driven to accept that the only way to prevent the loss of the northern white rhino will necessarily involve the resources of the Frozen Zoo.

Genetic material from 12 northern white rhinos is banked in the Frozen Zoo.

It is a long and improbable road that brought the last female northern white rhino in the Western Hemisphere, Nola, from the grassy swamps of the headwaters of the Nile, via the Khartoum Zoo and Eastern Bohemia Zoo in Czechoslovakia to the San Diego Zoo Safari Park, where I recently was able to watch and listen to her eat her breakfast. The satisfying sound of her chewing is a sound that, like the species itself, faces extinction, I reflected. Perhaps even more improbable is that her frozen cells will contribute to rescuing the northern white rhino from extinction. Yet, we are resolute to try.

Nola is the last northern white rhino in North America.

Since the first moment I learned about the existence of the northern white rhino, the question of their difference from the now more numerous southern white rhino was at the forefront. Legendary South African conservationist Ian Player, the man who led the effort to bring southern white rhinos back from a small and vulnerable population that was reduced in number to less than 100 to, now, the most numerous form of rhinoceros, posed the question the first time we met. It was another legendary individual, Dr. Kurt Benirschke, the founder of the conservation research effort at the San Diego Zoo, who had brought us together. With Dr. Benirschke’s support, a postdoctoral scientist, Matthew George Jr., conducted the first genetic studies comparing northern and southern white rhinos and published the findings in 1986. Since his initial studies, our own efforts and those of other investigators have added to our initial findings. All the studies provide evidence that the two forms are genetically diverged, but the methods used over the years have now been superseded by advances in genome sequencing that have taken place over the last decade.

Dr. Oliver Ryder holds a tissue sample from the Frozen Zoo.

Comparison of the sequenced genomes of northern white rhinos with southern white rhinos will provide an objective assessment of the divergence of the genomes of the two rhino forms. This “crash” of data will shed light on the question of whether they are sufficiently divergent to be considered species or subspecies. Whatever the revelation on this matter, it will be overshadowed by the detailed knowledge of the DNA sequences encoding their behavioral and ecological adaptations that have evolved since their divergence from a common ancestor, and the time frame over which these changes took place. The ability to resolve these and other questions is a hallmark of the entry into the era of genomic biology, and serves as an example of how this emerging science can contribute to conservation of biological diversity. Knowledge of the northern white rhino genome and its expression will, as we strive to turn the cells of northern white rhinos in the Frozen Zoo into young rhinos, serve as roadmaps for our efforts.

Oliver Ryder is director of the Genetics Division at the San Diego Zoo Institute for Conservation Research.

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